The present invention relates to light emitting semiconductor devices and methods of producing the same. More particularly, the present invention relates to light emitting semiconductor devices having reflection layer structure and methods of producing the same.
Conventional surface-emitting type of light-emitting devices (LED) are manufactured by using a light-absorbing substrate. FIG. 1 illustrates the structure of a prior art surface-emitting type LED, wherein a lower cladding layer 21 is formed on the light-absorbing substrate 20 first, and then an active layer 22 is formed on the lower cladding layer 21, and an upper cladding layer 23 is subsequently formed on the active layer 22, so as to form a double heterostructure. The wavelength of the light emitted from such an LED is dependent on the ratio of the composition of the active layer. The energy gap of each cladding layer is higher than that of the active layer, such that not only the carrier injection rate can be increased but also so that the light emitted from the active layer will not be absorbed by the cladding layers. Finally, a front metal electrode 24 is coated on the light-emitting surface of the LED, and a rear electrode 25 is coated on the surface of the substrate 20 opposite to the surface on which the double heterostructure is formed. Since such a vertical type LED uses a light-absorbing substrate, for example, a GaAs substrate, which can absorb the light with wavelengths from 570 nm to 650 nm, its light-emitting efficiency will be reduced. Therefore, how to overcome the light absorption problem caused by the substrate is a key to improve the light-emitting efficiency of such an LED.
To overcome the shortcoming of light absorption caused by the light-absorbing substrate and to improve the light-emitting efficiency of prior art LEDs, an alternative conventional structure, as illustrated in FIG. 2, has been provided by adding a current-block region 34 and a Bragg reflector layer 33. The current-block region 34 is formed on the upper cladding layer 23 and is of the same material as that of the upper cladding layer 23 but has different doping type. The current-block region 34 can increase the current-spreading area, and thus improve the light-emnitting efficiency. The Bragg reflector layer 33 which is formed between the light-absorbing substrate 20 and the lower cladding layer 21 can reflect the light directed to the light-absorbing substrate 20 and thus increase the light-emitting efficiency. However, in such a conventional structure, a further process should be used to define the area of the current-block region 34 and double MOCVD epitaxial processes should used to form the current-block region 34. Therefore, its manufacturing process will be complicated and the manufacturing time thereof is very long. In addition, the Bragg reflector layer 33 is manufactured by iterately stacking a pair of two different layers with different refraction indexes. The range of the reflection angle of the Bragg reflector layer depends on the difference between the refraction indexes of the two layers of the pair. However, since the material of the pair is restricted to be a compound semiconductor, the difference between the refraction indexes of the two layers of the pair is very limited, and thus merely the almost vertical incident light can be reflected by the Bragg reflector layer, while other incident light can pass the Bragg reflector layer and be absorbed by the substrate. Therefore, its effect of avoiding light from being absorbed by the substrate is very limited.
Another conventional structure, as illustrated in FIG. 3, has been provided, wherein the heterostructure 36 of an LED is formed on a temporal light-absorbing substrate 20 to meet the requirement of lattice match, and after the formation of the heterostructure 36 is completed, the temporal substrate 20 is removed, and then a transparent conductive substrate 35 is attached to the heterostructure 36 by using the technique of thermal wafer bonding. The transparent conductive substrate 35 will increase the current-spreading area and will not absorb the light emitted from the active layer, and thus will increase the light-emitting efficiency. In such a conventional structure, the concept of the thermal wafer bonding technique which is used to combine the heterostructure 36 with the transparent conductive substrate 35 is that the difference between the thermal expansion coefficients of two different materials will generate a single axis press, during a thermal process, which will force the generation of the binding, caused by atom-to-atom Van der Waals"" force, between the heterostructure 36 and the transparent conductive substrate 35. To achieve uniformity throughout a large area, it must generate uniform single axis press over a large area. Therefore, not only the thermal bonding machine should be specially designed, but also, that the surfaces of the transparent conductive substrate 35 and the light emitting heterostructure 36 must be of the same lattice direction so as to obtain enough bonding force and low resistance on the bonding surfaces thereof. Therefore, such a conventional method is very complicated and very difficult in manufacturing, and thus its yield rate is hard to increase.
Furthermore, a conventional gallium nitride-based light-emitting device using sapphire substrate must be manufactured as a lateral device, as illustrated in FIG. 4, for the reason that the sapphire substrate is insulated. Its structure comprises a sapphire substrate 40 on which a buffer layer 41, an n type lower cladding layer 42, an active layer 43, a p type upper cladding layer 44 and a p type ohmic contact layer 45 are formed serially as well as a front electrode 46 and a lateral rear electrode 47 which are formed subsequently. Silicon carbide has also been used as a substrate for a conventional gallium nitride-based light-emitting device. Although silicon carbide is conductive and the light-emitting device using silicon carbide substrate can be made to have vertical electrodes, silicon carbide is hard to manufacture and the cost thereof is very high. Conventional light-emitting devices using insulated substrates cannot be made to have traditional vertical type electrodes but must be of a lateral electrode structure. Therefore, not only special wiring mechanisms and special packaging techniques are needed, but also the area of a die is increased, so that the manufacturing process thereof will become very complicated and the cost for each unit is increased.
In view of the above, the shortcomings of prior art techniques are as follows:
1. adding a current-block region needs complicated MOCVD epitaxial processes, and the Bragg reflector layer can only reflect the light with an incident angle within a specific range;
2. the process of thermal wafer bonding to achieve uniform bonding and low resistance in the bonding interface is very complicated and difficult; and
3. gallium nitride-based light-emitting devices using sapphire substrates cannot be made to have vertical type electrodes, and thus increase the cost for each unit.
An objective of the present invention is to overcome the reduction of light-emitting efficiency by adapting a light-absorbing substrate.
Another objective of the present invention is to provide a method for easily combining a substrate with a light emitting semiconductor structure so as to reduce the complexity and difficulty of the manufacturing process and greatly increase the yield rate.
A further objective of the present invention is to simplify the manufacturing process of the current-block region and provide an effective current-spreading effect to improve light-emitting efficiency.
A still further objective of the present invention is to provide a manufacturing process which can easily convert the semiconductor light-emitting device with lateral electrode structure to be the one having a vertical type electrode structure so as to effectively reduce the area for an LED die and facilitate subsequent wiring and packaging processes using traditional mechanisms.
A light-emnitting device according to the present invention comprises
a semiconductor stack structure for generating light in response to a conduction of current;
a reflection layer provided on a main surface of the semiconductor stack structure for reflecting light generated from the stack structure and directed to the reflection layer;
a thick layer positioned above the reflection layer and functioning as a substrate; and
electrode structure for applying a current to the semiconductor stack structure.
The reflection layer may comprises at least one area with inferior conductivity functioned as a current-block region.
In addition, according to the present invention, a method for manufacturing a light-emitting device comprises the steps of:
forming on a first substrate a first conductive type lower cladding layer;
forming a second conductive type upper cladding layer adjacent to the lower cladding layer;
forming an ohmic contact layer above the upper cladding layer;
forming a reflection layer on the ohmic contact layer;
combining a second substrate with the reflection layer;
removing the first substrate; and
forming electrodes which are electrically coupled to the lower cladding layer and the upper cladding layer respectively.
In the light-emitting device according the present invention, the light-absorbing substrate has been removed during the manufacturing process, and thus totally overcome the problem of reduction of light-emitting efficiency caused by using a light-absorbing substrate. Further, the light-emitting device, according to the present invention, has a reflection layer which can effectively reflect the light directed to the substrate, and thus can improve the light-emitting efficiency of a surface-emitting type LED. The reflection layer of the present invention can comprise a single metal layer or multiple metal layers which can function as an intervention layer for facilitating the combination of the second substrate and the light emitting semiconductor structure. Therefore, not only the difficulties of conventional thermal wafer bonding can be avoided, but also the limitation to the choice of the material of the second substrate can be lessened. Moreover, in the light-emitting device, according to the present invention, the current-block region can be formed during the process of forming the reflection layer, and thus greatly simplify the manufacturing process of the current-block region.
In addition, the method of the present invention can be directly used to make a light-emitting device which originally utilized an insulated substrate to be one of having a vertical type electrode structure. Therefore, not only the wafer area used for a die can be effectively reduced, but also the final stage of the manufacturing can use the traditional wiring and packaging process. Further, it can provide a cleavage to divide the dies from each other, and thus it is suitable for making a laser diode.